The retaliatory attacks by pro-WikiLeaks activists are growing in strength as hackers add botnets and thousands of people download an open-source attack tool, security researchers said today.

In recent days, distributed denial-of-service (DDoS) attacks have been launched against several sites, including those belonging to Amazon, MasterCard, PayPal and the Swiss payment transaction firm PostFinance, after each terminated WikiLeaks accounts or pulled the plug on services.

As of Thursday, WikiLeaks had posted the full text of more than 1,200 leaked U.S. State Department cables from its trove of over 250,000 messages.

Most of those participating in the attacks are using the LOIC (Low Orbit Ion Cannon) DDoS tool, said researchers with Imperva and Sophos.

The open-source tool, which is sometimes classified as a legitimate network- and firewall-stress testing utility, is being downloaded at the rate of about 1,000 copies per hour, said Tal Be'ery, the Web research team lead at Imperva's Application Defense Center.

"Downloads have soared in the last two days," said Be'ery in an interview. As of 4 p.m. ET, more than 44,000 copies of LOIC had been downloaded from GitHub.

LOIC has become the DDoS tool of choice in the pro-WikiLeaks attacks because users can synchronize their copies with a master command-and-control server, which then coordinates and amplifies the attacks.

"If I download [LOIC] and voluntarily set the server information, the command-and-control server can control my copy of LOIC," said Be'ery. "The command-and-control server can then sync the attack, which makes it much more powerful because the DDoS attacks are occurring at the same time and hitting the same target."

Some will still want manually control LOIC, Be'ery said, calling those people "old school guys." But even then, the attacks are being coordinated.

"They're just syncing their attacks to the announcements made on Twitter and IRC (Internet Relay Chat)," Be'ery said, referring to the messages posted by several hacker groups, including Anonymous, which has been in the forefront of what's called "Operation Payback."

In a new step in the campaigns, botnets -- armies of already-compromised computers that hackers control remotely -- are now being recruited for the DDoS attacks, said Beth Jones, a senior threat researcher with Sophos. "Until now, the attacks have been done by volunteers who download LOIC," said Jones. "But now more groups are joining in with their botnets."

Be'ery said that Imperva had seen IRC chatter of at least one 100,000-PC botnet being thrown into the attacks.

"Operators of these attacks have repeatedly asked on IRC if someone can donate botnets," said Be'ery. "It looks like they feel the need for some more horsepower."

The fact that the organizers of Operation Payback are soliciting more firepower is a clue that they're not able to match the defenses erected by the sites they've targeted, said Be'ery. "They're having a bit of a problem. PayPal and others are doing good work to keep their sites alive, so they're after more machines and telling people [participating in the DDoS attacks] to do what they're told and focus on the targeted sites."

There seems to be something to Be'ery's point.

An attack launched earlier Thursday against Amazon.com by Anonymous appears to have fallen flat; the group then dropped Amazon and instead directed its PCs and followers to again hammer a PayPal URL.

But for all the problems that Operation Payback's having, Be'ery doesn't believe the DDoS attacks have peaked. "There doesn't seem to be any decay in the download rate of LOIC," he noted. "I really don't think things will change unless one of the attacked companies tries to take down the main command-and-control server."

There is only one such server currently coordinating the attacks, he added, but the organizers claim that they have a backup on stand-by. "But if the main server falls, it will certainly give them some trouble regrouping," said Be'ery.

Jones of Sophos saw a different end game.

"What's really surprising is that so many people are willing to put themselves on the line legally," she said, pointing out that using a tool like LOIC to attack a site is illegal in most jurisdictions, including the United States.

"A more firm legal response may be helpful," Be'ery agreed. "I'm not even sure that everyone understands that what they're doing is illegal."

On Wednesday, Dutch police arrested a 16-year-old in The Hague for allegedly participating in the attacks against Visa, MasterCard and PayPal. The teen is to be arraigned in Rotterdam on Friday.

"The penny will drop when some of these guys are arrested," predicted Be'ery.